As a device that estimates a state of a secondary battery and controls the secondary battery, for example, a control unit described in JP 2012-159414 A has been known. The control unit described in JP 2012-159414 A estimates the state of the secondary battery including a nonlinear area of a current-voltage characteristic, based on a battery model (hereinafter, referred to as a 1 RC equivalent circuit model) that includes one direct current (DC) resistance and one RC equivalent circuit.
According to JP 2012-159414 A, the 1 RC equivalent circuit model cannot express the characteristic in the nonlinear area. Therefore, the 1 RC equivalent circuit model sets a filter to attenuate a signal in the nonlinear area and make a calculation only with data in a linear area. As such, the 1 RC equivalent circuit model can manage the characteristic in the nonlinear area. It has been known that the nonlinear current-voltage characteristic of the secondary battery is more dominant with a temperature of the secondary battery reduces. Especially, in a temperature range of 0° C. or less, the nonlinear current-voltage characteristic is not negligible. Therefore, according to JP 2012-159414 A, since the data that can be used in the calculation is decreased in the temperature range of 0° C. or less, there is a possibility that the control unit cannot make an accurate calculation and an estimating accuracy of the state of the secondary battery is decreased. To address this possibility, it can be considered a control unit capable of estimating the state of the secondary battery accurately even when the secondary battery has a low temperature.